Zinc alloys

ABSTRACT

THE NEW ALLOYS COMPRISE BY WEIGHT ZINC WITHIN THE RANGE 70-82 PERCENT (THE REMAINDER BEING SUBSTANTIALLY ALUMINUM) AND MAGNESIUM WITH THE RANGE GREATER THAN 0.05 TO 0.25 PERCENT WHICH MAY CONTAIN UP TO 2 PERCENT OF ONE OR MORE OF THE ELEMENTS COPPER, NICKEL AND SILVER. THE MAGNESIUM IN THE ABOVE ALLOYS MAY BE REPLACED BY LITHIUM. AN EXAMPLE OF THESE ALLOYS CONTAINS ZINC AND ALUMINIUM WITHIN THE ABOVE PRESCRIBED LIMITS AND MAGNESIUM AND COPPER WITHIN THE RANGE 0.125$0.025 AND 1.25$0.25 PERCENT RESPECTIVELY. THE ALLOYS HAVE IMPROVED MECHANICAL PROPERTIES AND CORROSION RESISTANCE AND THE MICROSTRUCTURES PRODUCED

United States Patent AM 7 Claims ABSTRACT OF THE DISCLOSURE The new alloys comprise by weight zinc within the range 70-82 percent (the remainder being substantially aluminium) and magnesium within the range greater than 0.05 to 0.25 percent which may contain up to 2 percent of one or more of the elements copper, nickel and silver.

The magnesium in the above alloys may be replaced by lithium.

An example of these alloys contains zinc and aluminium within the above prescribed limits and magnesium and copper within the range 0.125 :0.025 and 1.25 i025 percent respectively.

The alloys have improved mechanical properties and corrosion resistance and the microstructures produced are stable at user temperatures. Additionally, the alloys retain the super plastic behaviour of the zinc-aluminium eutectoid.

This invention relates to novel zinc-aluminium based alloys.

Binary zinc-aluminium alloys based on the composition 78% zinc and 22% aluminium by weight are known. These alloys may be given a lamellar or non-lamellar microstructure by suitable heat treatment but are not used industrially because of the following disadvantageous properties:

(a) The rate of reaction during and after quenching is high thus causing limitation in the size of piece and in the control of resultant structures;

(b) Mechanical properties are insufliiciently attractive;

(c) Resistance to moisture attack is of a low order.

It has now been found that zinc-aluminium based alloys of superior and unexpected properties without the above disadvantages may be obtained by incorporating one or more of the elements magnesium, copper and lithium to the alloy. Specific properties of thenew alloys according to the invention may be further enhanced by the addition of minor quantities of nickel and/ or silver.

The new alloys according to the invention comprise by weight zinc within the range 7 082 percent (the remainder being substantially aluminium) and magnesium within the 7 range greater than 0.05 to 0.25 percent and which may contain up to 2 percent of one or more of the'elements copper, nickel and silver.

When the selected added element is copper, the alloys may conveniently comprise magnesium within the range greater than 0.05 to 0.20 percent by weight and the amount of copper lies within the range 0-1.0 percent by weight.

The magnesium in the above alloys may be replaced Patented July 11, 1972 rate of the binary zinc/aluminium eutectoid is slowed down, that is the rate of transformation of the rapidly cooled metastable a-phase into the equilibrium ufi mixture. Although magnesium, copper and lithium are eflective in this respect, magnesium is ten times more effective than copper magnesium over about 0.12 percent with or without copper, provides grain refinement by free particle formation.

The known susceptibility of the binary zinc/ aluminium alloys to moisture attack is markedly reduced by the addition of one or more of the elements magnesium and copper. This susceptibility is reduced to about one-tenth of its value by the addition of about 0.1 percent of magnesium or 1.0 percent of copper. In combination, the addition of magnesium and copper reduces the susceptibility to almost negligible proportions.

Lithium in addition to reducing the reaction rate also improves the resistance to moisture attack.

Nickel combines with aluminium to provide particles of an independent microstructural phase and may be used to provide grain refinement particularly in alloys with a low magnesium content.

Silver may be used, in part, as a substitute for copper.

Many alloy combinations, according to the invention, for instance those containing both magnesium and copper, can be made by suitable heat treatments to provide tensile strengths in the range 25.30 tons f./ sq. in., with yields of 20-25 tons and elongations over 15 percent. In such combinations, the addition of copper improves the subzero impact values.

Zino/ aluminium alloys containing both magnesium and copper have outstanding overall properties when the proportions of magnesium and copper lie within the range 0.125 $0.025 and 1.25 :0.25 percent respectively. Variations in the composition, both in the alloying element or elements added and in the proportions thereof, may be made to emphasize special properties such as control of reaction rate, for example to provide adaptability in producing a desired end condition by means of specific production cycles.

Variations in the proportions of zinc and aluminium from the eutectoid composition (78 percent zinc, 22 percent aluminium) provide alloys with marginally inferior properties with no deleterious unwanted characteristics. These changes in properties are, however, gradual and within the approximate range zinc 74 to 82 percent and aluminium 26 to 18 percent, the deterioration in physical properties of the alloys according to the invention is not detrimental provided the correct heat-treatments are used. Thus the chemical segregation which may normally be found in large ingots will not present difliculties.

Alloys containing zinc in lower proportions, down to percent, are not so strong. Their rate of response to a coarsening heat-treatment is more rapid than for eutectoid alloys and they can, therefore, be converted more economically into a granular state. These low zinc content alloys may be suitably used as a weaker alloy with good low temperature impact properties.

The heat treatment of the novel alloys consists of homogenising the alloy by heating with the a zone (280 380 0), followed by cooling at a predetermined rate with or without subsequent reheating at temperatures up to 275 C. The predetermined rate of cooling may consist of slow cooling through the eutectoid change, or a sufficiently rapid cooling down to ambient temperature or below to suppress eutectoid transformation or a rapid cooling down only to the re-heating temperature. The precise cooling conditions may be determined by the microstructure required, for instance a slow cooling can give a lamellar structure.

An unusual and advantageous property of the novel alloys is that the microstructures produced are stable at user temperatures. An alloy containing 78% zinc, 22% aluminium and 0.15% magnesium was treated at 250 C. for 4 hours. After standing at room temperature for 77 days, it showed no change in hardness. In further tests, this magnesium alloy and the binary 78/22 zinc/aluminium alloy were treated at 250C. for 24 hours. The binary alloy slowly softened at room temperature from 73 Hv to 65 Hv in 70 days. The magnesium containing alloy not only did not soften at room temperature but its superior hardness value, 109 Hv, was not affected by holding for 20 hours at 103 C.

Plastic deformation of the alloys may be undertaken by standard procedures e.g. rolling and extrusion. After deformation, the alloys can, if so desired, be heat treated to attain their improved mechanical properties.

The unusually high values of elongation denoting what is known as superplasticity are known to be associated with microstructures of particularly fine texture. This superplastic behavior displayed by the binary alloys of zinc and aluminium is not lost when the alloying element is present; in fact, its range of usefulness may be extended by control of the alloy composition and the temperature use. For instance, an alloy containing 78% zinc, 22% aluminium and 0.15 magnesium pretreated 2 hours at 360 C. and then isothermally for 2 minutes at 188 C. When tested at a strain-rate of 0.01 inch/minute and a temperature of 250 C. it had a yield point of 1.14 tons f./sq. in. and an elongation of 450% The alloys according to the invention have the following advantages:

(1) The reaction rate of the binary zinc/aluminium eutectoid is slowed down. This rate reduction permits effective quenching of thicker sections or lower cooling speeds for the same section. For example, binary alloys of /2 inch diameter must be water quenched whereas the addition of 0.15 magnesium permits air cooling in this size.

(2) Improved mechanical properties such as U.T.S., Hardness, Creep Strength and Impact Strength in the user range -40 C. to +50 C.

(3) High quality surface finishes are achieved if suitable procedures for hot working and heat treatment are carried out.

(4) Improved corrosion properties.

(5) The new alloys have good machinability.

tone f. per sq. in. The novel heat treated alloy had an unnotched Houndsfield impact value of 30ft. lb. with a transition temperature about 80 C. It was resistant to steam attack and had an electrical conductivity of 28.4 percent I.A.C.S. (International Annealed Copper Standard (20 C.) This electrical conductivity is comparable with that of 70:30 brass which alsohas a conductivity of 28 percent I.A.C.S. when rolled to this hardness level l 14 Hv) A further alloy was made of the following composition, zinc 78% and aluminium 22% by weight to which 0.055% lithium was added, by adding with stirring the requisite amount of a master alloy consisting of 95% aluminium, 5% lithium to a binary zinc/aluminium alloy. This prepared alloy, after undergoing the heat treatment procedure previously described, hada hardness of 95 Hv.

The properties of further alloys made in accordance with the invention are given in the following tables.

Table I shows the mechanical properties of zinc/aluminium/magnesium alloys containing varying amounts of zinc. 1

TABLE I 1 Heat treatment, 2 hours at 360 (3., water quenched, and 1 hour at Composition, percent by weight Yield Alumin- Other point, U.T.S., Elongation,

Zinc ium elements t.f./sq. in. t.f./sq. in. percent 70 30 +0.15 Mg 17 11. 4 Ca. 10

Table II shows the Charpy Impact Values at ambient and subtemperatures of zinc/aluminium/magnesium/ copper alloys.

TABLE II Table III shows the mechanical properties of alloys containing 78 percent zincand 22 percent aluminium by Weight with varying proportions of magnesium and copper.

TABLE III Quenehed from 360 0., ire-heated for 2 hours at 260 C. and

water-cooled Furnace-cooled from 360 C. at 30 0. per hour Avlitglaorsnogitzigrllliy Tensile strength. t./sq. in. Imarfigt 155 2 1 1 1 Tensile strength, t./sq. in. V lvmapggt with 11v Y.P. U.T.S. E percent ft.-1b. Hv Y.P. V U.T.S. E percent it.-lb.

0.15% Mg 118-122 22.6 26.2 16.7 91-92 20.2 22.6 8.7 0.15% Mg, 1.0% Cu 119-128 25.4 28.3 15.5 98-99 21 23. 5 14.2 0.10% Mg, 05% C11 118 23.6 27. 5 19. 8 C. 105 96. 5 20 22.6 14. 2 60 C. 40 0.20% Mg, 0.5% Cu 117-121 24.2 28.6 17.9 60 0.60 94-100 19.5 22.9 11.7 jg: 8:

*Unnotched Charpy specimens.

By way of example of the novel alloys according to the invention an alloy of the following composition was made: Zinc 78 percent and aluminium 22 percent by weight to which 0.15 percent of magnesium was added. The alloy was prepared by melting a binary zinc/aluminium alloy and adding with stirring the requisite amount of a master alloy consisting of 95 percent aluminium, 5 percent magnesium. The alloy was quenched from a temperature of 360 C. by immersing it in water at 20 C. The quenched alloy was subsequently heated to a temperature of 250 C. for one hour. The heat treated alloy had a hardness of 114 Hv (Vickers) and an ultimate tensile strength of 25 tons f./sq. in. in with 8 percent elongation. By way of comparison, the binary alloy'78/22 zinc/aluminium when given a similar heat treatment had a hardness of 75 Hv and an ultimate tensile strength of 12 Table IV shows the comparative effect of magnesium and copper additions on the properties of a 78 percent zinc 22 percent aluminium by weight alloy. The results were obtained at room temperature on extruded stock which had been-subjected to a heat treatment consisting of being held 2 hours at 360 C; transferred to" 186 C.

Table V shows the tensile behaviour of extruded stock alloys at elevated temperatures in a pre-treatment condition and at a strain rate of 0.01 inch/minute. All the alloys contained 78 percent zinc and 22 percent aluminium by weight to which other alloying elements were added.

TABLE V Temper- Yield ature point, Elonga- Elements of test, t.f./ tion, added Pretreatment 0. sq. in. percent Plus 0.1% Mg, 2 hours at 360 0. plus 150 7. 7 90 2.0% Cu. isothermally for 2 hrs. 200 2. 4 200 at 260 0. Plus 0.1% Mg, do 150 5. 7 110 2.0% N l. Plus 0.1% Mg, '..---d0.- 150 5. 5 133 0.5% Ag. Plus 0.1% Mg, 2 hrs. at 360 0., water 150 5. 6 166 1.0% Ni. quenched then 16 hrs. 200 2. 3 200 at 125 0. plus 4 hrs. at 260 0. Plus 0.15% Mg.. 2 hrs. at 360 0. plus 250 1. 14 450 isothermally for 2 mins. at 188 0.

Alloys according to the invention were subjected to steam at 100 C. for periods of time. Microsections of the samples were examined at a magnification of X500 at the conclusion of the tests to determine the depth of penetration of the attack.

The alloys tested contained 78% zinc, 22% aluminium by weight to which was added 0.10% Mg, 0.5% Cu+0.10 to 0.20% Mg, 1.0% Cu+0.1% Mg, 1.0% Cu+0.15% Mg. 2.0% Cu+0.1% Mg, and 2.0% Cu+0.15% Mg were added.

All the alloys were pretreated at 360 C. for 2 hours plus 1 hour isothermal at 188 C. plus 1 hour at 260 C. and were then subjected to steam at 100 C. for 48 hours. The results are shown in Table VI.

TABLE VI Approximate depth of Elements added: penetration, inches Further tests carried out for 96 hours confirmed these comparative results.

By comparison, an alloy containing 0.10%. Mg when exposed out of doors for 1 year showed an extremely slight attackapproximately 0.001 inch.

What we claim is:

'1. A wrought, superplastic, two-phase alloy having a fine grain microstructure stable at the temperature of superplastic deformation comprising zinc within the range -82% by weight, aluminum in the range of 18-30% by weight, magnesium in proportions from 0.05 to 0.25% by weight and up to 2% by weight of one of the elements copper, nickel and silver.

2. A wrought, superplastic, two-phase alloy according to claim 1 in which the proportion of magnesium is between 0.10% and 0.15%.

3. A wrought, superplastic, two-phase alloy according to claim 1 wherein magnesium is in the proportion of 0.125-0.025% and copper is in the proportion of 1.25- 0.25%.

4. A wrought, superplastic, two-phase alloy according to claim 1 wherein the magnesium is in a proportion of 0.05 to 0.2% and copper is present up to 1%.

5. A wrought, superplastic, two-phase alloy according to claim 4 wherein the proportion of magnesium is within the range of 0.1 to 0.15%.

6. A process for the preparation of alloys according to claim 1 comprising subjecting the cast alloy to a homogenising treatment at a temperature in the range of 280- 380 C., cooling said alloy, reheating the homogenized alloy to a temperature in the range 20275 C. and plastically deforming the reheated alloy in the temperature range of 20-275" C.

7. A process according to claim 6 wherein the plastically deformed alloy is subsequently reheated in the temperature range of 280380 C. to enhance its strength, creep resistance and ductility.

References Cited UNITED STATES PATENTS 1,945,288 1/1934 Morell 178 AM 2,008,529 7/ 1935 Werley 75-178 AM 3,420,717 1/ 1969 Fields et a1. 1481l.5

FOREIGN PATENTS 873,336 7/1942 France 75178 AM 335,270 2/ 1936 Italy 75-178 AM 512,758 11/1937 Great Britain 75178 AM 5,659 2/ 1954 Germany.

L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. l48--ll.5, 32

UNITED STATES PATENT OFFICE CERTiFl CA'lE 0F CCRRECHCN Patent No, 3, 7 5 Dated July 11, 1972 lnventofls) George Anthon Hare and Alan Ernest William Smith It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 3, read -cooled meta-stable CU phase into the equilibrium r-fl mix- Column 2, line 6, read than copper. Magnesium over about 0,12 percent with or-.

Column 2, line 27, read -sile strengths in the range 25-30 tons i'./sq. in,, with yields--.

Column 2, line 60, read --homogenising the alloy by heating with the wzohe (28o- Column 3, line 2 r, read -aluminium and 0.15% magnesium was pretreated 2 hours at",

Table III, line l, last sub-column, read -Impact*- Claim 3, line 3, read "0.125? 0.025% and copper is in the proportion of 1.25 1' Signed and sealed this 15th day of May 1973.

(SEAL) Attest:

EDWARD MQFLETCHERJRQ I ROBERT GOT'ISCHALK Attesting Officer I v Commissioner of Patents FORM P0-1050 (10-65) USCOMM-DC 60376-P69 Q U,5. GOVERNMENT PRINTING OFFICE I9! 0-366-33 

